1. Field of the Invention
The present invention relates to a particle inspection apparatus that detects a particle adhering to an object to be inspected, for example, an original.
2. Description of the Related Art
In exposure apparatuses, a circuit pattern used to manufacture a device, for example, a semiconductor device such as an IC or an LSI, a CCD, a liquid crystal panel, or a magnetic head (hereinafter generically named a “device”) is provided on an original called a reticle or a photomask.
A pellicle serving as a protective cover is attached to an original so as to prevent a particle from adhering onto the original.
In a normal IC manufacturing process, a circuit pattern provided on an original, such as a reticle or a photomask, is transferred onto a resist-coated wafer by an exposure apparatus (stepper or mask aligner).
In this case, if a particle, such as a pattern defect or dust, is present on the original, the particle is also transferred simultaneously. This lowers manufacturing yields of ICs or LSIs.
Particularly when a circuit pattern is repeatedly transferred into multiple regions on a wafer by a step-and-repeat method using a reticle, if one harmful particle is present on the reticle, the particle is transferred onto the entire wafer. As a result, the yields of ICs and LSIs are lowered significantly.
Therefore, it is essential to detect the presence of a particle on the original in the IC manufacturing process. To this end, most exposure apparatuses incorporate particle inspection apparatuses.
A particle inspection apparatus conducts a particle inspection on an inspection region preset on the original around the start of pattern transfer in the exposure apparatus.
A particle inspection apparatus of the related art incorporated in the exposure apparatus will be described with reference to FIGS. 6A and 6B serving as schematic structural views.
A detection unit 20 includes a semiconductor laser 11, a condenser lens 13, half mirrors 14 and 22, and image sensors 19a and 19b. 
The detection unit 20 is driven relative to a reticle 15 by a driving unit (not shown), and can detect a particle 17 over the entire surface of the reticle 15.
Inspection light 12 emitted from the semiconductor laser 11 is shaped into parallel light by the condenser lens 13, and is divided into two inspection light beams 12a and 12b by the half mirror 14. The inspection light beam 12b is then reflected by the half mirror 22.
Further, the inspection light beams 12b is applied onto the reticle 15 serving as an object to be inspected, and a pellicle 16 serving as a protective cover for a circuit pattern 23. The pellicle 16 is held on the reticle 15 by a pellicle frame 21.
When the particle 17 is present in an irradiation region irradiated with the inspection light beam 12b, scattering light 18 is generated and is received by the image sensor 19b provided in the irradiation region, so that the particle 17 is detected.
In the exposure apparatus whose installation space is considerably restricted, the particle inspection apparatus incorporated therein often inspects both a front surface and a back surface of the reticle 15 by using the half mirrors 14 and 22 that divide the inspection light 12 into the inspection light beams 12a and 12b, as in the particle inspection apparatus shown in FIGS. 6A and 6B.
Japanese Patent Laid-Open No. 11-83752 proposes a surface-state inspection method and a surface-state inspection apparatus in which ranks of particles are reasonably set according to actual properties of the particles, and which eliminate the necessities of cleaning a reticle and of replacing a pellicle and reduces the risk of missing a particle.
Japanese Patent Laid-Open No. 8-15169 proposes a particle inspection apparatus that can precisely detect a particle, such as dust, adhering to a surface of a reticle or a pellicle, and a semiconductor-device manufacturing method using the inspection apparatus.
FIG. 7A shows a state in which a part 33 of the inspection light beam 12a entering the reticle 15 from an end of the reticle 15 is diffracted by the circuit pattern 23 and is incident as diffracted light 34 on the image sensor 19b. 
FIG. 7B shows a state in which the influence of the diffracted light 34 appears in an output 25 from the image sensor 19b. 
The part 33 of the inspection light beam 12a enters the reticle 15 from the end, reaches the circuit pattern 23 in a circuit pattern region 24 shown in FIG. 6, and generates the diffracted light 34 from the circuit pattern 23 on a side opposite the incident direction (a lower side of the reticle 15). When the diffracted light 34 enters the image sensor 19b, a particle scattering-light signal 26 and a pattern diffracted-light signal 27 appear in the output 25 of the image sensor 19b, and the diffracted light 34 is erroneously detected as the particle 17.
Intense diffracted light 34 that affects the particle inspection is generated by interference of the diffracted light 34 itself. For this reason, the direction in which interference light of the diffracted light 34 (hereinafter simply referred to as diffracted light) is generated is determined by, for example, the repetition pitch of the circuit pattern 23.
Hence, in the related art, the diffracted light 34 is prevented from entering the image sensor 19b by adjusting the positions of the image sensors 19a and 19b. 
However, since the integration density of ICs has been increased and the circuit pattern has been diversified in recent years, it is difficult to avoid all diffracted light 34 by simply adjusting the positions of the image sensors 19a and 19b. 
For this reason, the diffracted light 34 sometimes enters the image sensor 19b, and is erroneously detected as the particle 17.
When detection error is caused by the diffracted light 34, the reticle 15 is subjected to a cleaning process. This reduces the operating rate of the exposure apparatus.
While the inspection light beam 12a enters the reticle 15 from an upper end of the reticle 15 in the related art shown in FIG. 7A, the above also applies to cases in which the inspection light beam 12a enters from a front surface of the reticle 15 or a front surface of the pellicle 16.